Mixed oxide containing a lithium manganese spinel and process for its preparation

ABSTRACT

A mixed oxide containing
     a) a mixed-substituted lithium manganese spinel in which some of the manganese lattice sites are occupied by lithium ions and   b) a boron-oxygen compound.   

     Furthermore, a process for its preparation and the use of the mixed oxide as electrode material for lithium ion batteries.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Phase application of PCT application numberPCT/EP2009/004512, filed Jun. 23, 2009, which claims priority benefit ofGerman application number DE 10 2008 029 804.2, filed Jun. 24, 2008, thecontent of such applications being incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a mixed oxide containing a mixed-doped lithiummanganese spinel and a boron-oxygen compound as well as a process forthe preparation thereof and its use, in particular as cathode materialin lithium-ion secondary batteries.

BACKGROUND OF THE INVENTION

Rechargeable lithium-ion batteries are characterized by high specificenergy and power. They are therefore preferably used in applicationswhich require the smallest possible weight and little space. Thus, it islithium-ion secondary batteries that are predominantly used today asenergy stores for portable electronic devices, such as e.g. videocameras, laptop computers or mobile phones. If the production costs ofrechargeable lithium batteries are further reduced, further potentialapplications are conceivable, such as e.g. cordless power tools, onboardpower supply, traction or hybrid batteries in vehicles, or also instationary applications, e.g. for emergency power supply equipment.

The operating principle of current lithium-ion batteries is based on theuse of electrode materials which can intercalate lithium reversibly. Atpresent, carbon compounds are customarily used as anode and alithium-containing oxide as cathode.

In order to be able to obtain the highest possible energy densities,cathode materials which can intercalate lithium at potentials between 3and 4 V vs. Li/Li+ are preferably used. The most promising materialswhich meet these requirements include lithium compounds based on cobalt,nickel, iron and manganese oxides. For reasons of cost and because theyare more environmentally friendly and safer to use, manganese- andiron-based materials are currently preferred.

Of the lithium manganese oxides, compounds with spinel structure, suchas e.g. LiMn₂O₄ (stoichiometric spinel), Li₂Mn₄O₉ (oxygen-rich spinel)and Li₄Mn₅O₁₂ (lithium-rich spinel), show the most promising propertiesas cathode materials. Normally, these spinels are prepared by means ofsolid-state reactions. This normally results in stoichiometric spinelswhich, however, in most cases have only an inadequate cycle life. Thisis attributed to changes and defects in the lattice structure whichoccur during the insertion and removal of lithium ions.

DESCRIPTION OF RELATED ART

In U.S. Pat. No. 4,507,371 an electrochemical cell is described in whichthe anode, electrolyte and cathode each have a material which has acubic-close-packed framework structure, which has, as its basicstructural unit, a unit of the formula (B₂)X₄ ^(n−) and is thestructural unit of an A(B₂)X₄ spinel. The structure accommodates activecations M⁺ capable of diffusion within this structure. This documentalso describes the forms of crystal defects and stoichiometricdeviations known in structural chemistry for spinel structures of theAB₂X₄ type, such as for example the partial replacement of B-cations byA-cations or the lack or excess of X-anions, and teaches that these canexert an influence, not quantified in more detail, on theelectrochemical properties.

U.S. Pat. No. 5,316,877 describes spinel compounds of the generalformula Li_(l)D_(x/b)Mn_(2−x)O_(4+δ) with 0≦x<0.33; 0≦δ<0.5; wherein Dis preferably Li, Mg, Co, and b is the oxidation state of this cation.The described embodiment examples are limited to the case where D=Li.The preparation is by means of a solid-state process in which LiNO₃, orLiOH is mixed with γ-MnO₂ in the desired molar ratio together withhexane in a ball mill. Then the mixture is fired in air for 48 h at 450°C. and fired for a further 48 h at 750° C.

In EP 0 783 459 B1 doped lithium complex oxides of the general formulaLi_(y)Me_(x)Mn_(2−x)O₄ are described which have a spinel-typecrystalline structure and are suitable as cathode material in lithiumsecondary batteries. The batteries produced with these cathodes are saidto have a long cycle life. The lithium manganese spinel is prepared byreacting quantities, corresponding to the stoichiometry of the spinel tobe prepared, of the reaction components in the form of hydroxides and/orwater-soluble metal salts in dissolved form in a basic aqueous mediumaccompanied by formation of a homogeneous suspension. The dried reactionproducts are then heated to temperatures of between 500 and 900° C. at aheating rate of 1-20 K/min, wherein the respective mixed oxides form inradiographically phase-pure form. Particularly suitable metal cations Meare iron, titanium and aluminium, as well as cobalt and nickel. Complexoxides of the formulae LiFe_(0.1)Mn_(1.9)O₄, LiTi_(0.1)Mn_(1.9)O₄ andLiNi_(0.1)Mn_(1.5)O₄ are disclosed in the examples. The patent teachesthat a uniform fine distribution of the starting components is importantfor the preparation of phase-pure doped lithium manganese spinels andthat this fine distribution is achieved particularly advantageously bythe joint precipitation of the starting components from a solution.

In EP 1 035 075 A1, a two-stage precipitation and suspension process forthe preparation of a complex lithium manganese spinel doped inter aliawith boron is described. The compound has the following general formula:

Li_(x)Mn_((2−y))M_(y1)B_(y2)O₄,

wherein M is selected from the group of Al, Cr, Fe, Ni, Co, Ga and Mg,and furthermore: 0.9≦x≦1.1; y=y₁+y₂, wherein 0.002≦y≦0.5 and 0≦y₁<0.5and 0.002≦y₂≦0.1. The doping element boron is added in the secondprocess step because of its ready solubility and inserted in the latticesites of the manganese.

Robertson et al. (J. Electrochem. Soc. 144, 10, (1997), 3500-3512)report enlarged crystallites (about 1 μm) and a reduced specific surfacearea according to BET (<1 m²/g) for a boron-doped spinel of thecomposition LiMn_(1.98)B_(0.02)O₄. The associated scanning electronmicroscopy image shows large octahedral crystallites with roundedcorners and edges with indicated crystal surfaces. However, very poorelectrochemical properties are also reported for this material. Thus, aspecific capacity of less than 50 mAh/g was measured, which alsodecreases markedly from cycle to cycle.

In EP 1 204 601 B1, lithium manganese spinels for thin-film electrodesare described. In the process of EP 1 204 601 B1, the obtained lithiummanganese spinel is sintered in the form of agglomerates in oxidizingatmosphere at 450° C. to 900° C., wherein 0.1 to 3% boron oxide or boricacid, relative to the weight of the solids used, can be added assintering auxiliary, in order to achieve a higher strength of theagglomerates. No effect of the boron compounds on the crystallinity ofthe spinel is described.

In EP 0 993 058 A1, a lithium secondary battery is described, thepositive electrode active material of which is composed mainly oflithium and manganese and has a cubic spinel structure. In a comparisonexample, LiMn₂O₄ doped with boron is used as electrode material. Thiselectrode material is prepared by mixing powdery Li₂CO₂, MnO₂ and B₂O₃in the molar ratio Li:Mn B=1:2:0.03 and calcining the obtained mixturein an oxidizing atmosphere for twenty-four hours at 800° C. Particlesare obtained which have a deformed octahedral shape with flat crystalfaces, wherein the edges and corners of the crystals are rounded.

In JP 2001-48545, a process for the preparation of a complex boron-dopedlithium manganese oxide with spinel structure is described, wherein alithium compound and a manganese compound are reacted in liquid phase inthe presence of a boron compound. The doping element aluminium isintroduced in the form of a solid compound. Roughly spherical granulatedparticles with an average particle diameter of 15 μm are obtained.

EP 1 136 446 A2 discloses coated lithium complex oxide particles and amethod for their preparation. The coating, which can be formed as asingle or as several identical or different layers, is said to improvethe properties of electrochemical cells in which a cathode consists ofthe coated lithium manganese oxide material according to aspects of theinvention. Although doping with additional lithium (hyperstoichiometriclithium) is described in the overall disclosure, it is not explicitlydisclosed. Typical coatings are obtained with alkali borates and boronlithium complex salts.

P. Strobel et al. describe, in “Cation Ordering in Substituted LiMn₂O₄Spinels”, Mat. Res. Soc. Symp. Vol. Proc. 756, the effect of differenttrivalent and divalent metal cations as doping agents which occupy the16d manganese lattice sites on the electronic properties of such dopedlithium manganese spinels. In particular, the replacement of precisely aquarter of the octahedral cations (16 e) was attempted here and it wasshown that, for magnesium and zinc, this resulted in a superstructurewith a primitive cubic symmetry. In the case of zinc, a further latticedistortion was detected at the tetrahedral 8a site by FTIR spectroscopy.

DE 199 13 925 A1 discloses hyperstoichiometric lithium manganesespinels. The lithium manganese spinels in this patent applicationconform to the general formula Li(Mn_(2−x)Li_(x))O₄, wherein x is0<x≦0.8, which can be obtained by a spray pyrolysis process. Lithium isinserted instead of manganese in its 16d lattice sites, but the lithiumdoping is limited to the range 0<x≦0.05, since outside of this range thecycle characteristic of cathodes containing this material deteriorates,because of the Jahn-Teller effect, compared to the unsubstituted lithiummanganese spinels.

U.S. Pat. No. 7,217,406 describes granular particles of alithium-manganese oxide for use as material in secondary batteries, inwhich boron, in particular H₃BO₃, is added as so-called “fluxing agent”.Boron is inserted instead of manganese in its lattice sites.

Buhrmester's doctoral thesis (Darmstadt 2001) deals with the defectstructure in the Li_(1+x)Mn_(2−y)O_(4−Δ) system and describes thestructural and electronic behaviour of lithium manganese spinelshyperstoichiometrically doped with lithium.

Lithium secondary batteries are also increasingly used for applicationsin which a long cycle life, a high current carrying capacity and a highsafety in the event of misuse or malfunction are required. Suchrequirements must be satisfied for example for storage batteries inhybrid vehicles which have an electric motor in addition to a normalcombustion engine. In addition, such batteries must also be cheap toproduce. The production costs are influenced by all components of thebatteries.

DESCRIPTION OF THE INVENTION

An object of the invention was therefore to provide material, inparticular a material containing a lithium manganese spinel, which canbe used as electrode material in particular in secondary lithium-ionbatteries and which has good charging characteristics.

This object is achieved by a mixed oxide containing

-   a) a mixed-substituted lithium manganese spinel in which some of the    manganese lattice sites are occupied by lithium ions and-   b) a boron-oxygen compound.

Surprisingly, it was found that the mixed oxide according to aspects ofthe invention has improved electrochemical properties compared with anon-doped lithium manganese spinel or one doped with foreign atoms suchas Zn, Mg, Cu, Al (mixed-substituted), but also compared with a lithiummanganese spinel only hyperstoichiometrically doped with lithium or sucha spinel which is doped with further foreign atoms.

The term “some” is explained in more detail in the following Formula Iby the numerical value for the index c.

The term “mixed oxide” describes according to aspects of the invention asingle-phase homeotype mixed crystal comprising the two constituents(e.g. Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie, 102^(nd)edition, de Gruyter 2007, pp. 130-131) and serves to illustrate that theboron of the boron-oxygen compound is not a doping element for the 16 dmanganese lattice sites in the lithium manganese spinel and does notoccupy these sites. In this case, the compound would be a so-called“complex” oxide.

The term “mixed-substituted” means that, in addition to(hyperstoichiometric) lithium, at least one further metal, other thanmanganese, is located on the 16 d lattice sites of the manganese.

Constituent a) of the mixed oxide according to aspects of the invention,the lithium manganese spinel, is typically also described as so-called“hyperstoichiometric” lithium manganese spinel.

In particular, the single-phase mixed oxide according to aspects of theinvention has a smaller BET surface area than comparable lithiummanganese spinels doped with lithium and/or with metals or transitionmetals or pure lithium manganese spinels, namely a BET surface area of<4 m²/g, preferably <2 m²/g and more preferably of <1 m²/g and mostpreferably <0.5 m²/g, whereby the manganese dissolution in theelectrolyte of a corresponding cell is clearly reduced, since the BETsurface area is correlated with the manganese dissolution in theelectrolyte.

The resistance to a dissolution in the electrolyte is thus increased bythe mixed oxide according to aspects of the invention, i.e. inparticular thus by the presence of the boron-oxygen bond (see below).

In addition, for the mixed oxide according to aspects of the invention,very narrow reflex widths in X-ray diffraction spectra are obtained,since according to aspects of the invention very large crystallites ofthe mixed oxide are surprisingly obtained. The large crystallites leadto a reduced surface area of the mixed oxide, whereby the dissolution inthe electrolyte is further reduced and the tap density (or, in otherwords, compression) is also increased. In comparison with a pure lithiummanganese spinel (LiMn₂O₄), an improvement in the tap density from 1.23g/cm³ to 2.03 g/cm³ is observed for the mixed oxide according to aspectsof the invention. By the term “tap density” is meant in the present casethe compression of the powder through exposure to the action of anexternal pressing force (“packing force”) of a sample of the respectivematerial, starting from an original bulk volume V_(o) of the sample. Theexternal pressing force acts on a container filled with a predeterminedquantity of the sample, which then repeatedly strikes a flat surface(see e.g. www.particletesting/com/density.aspx).

Moreover, the presence of the boron-oxygen compound in the mixed oxideaccording to aspects of the invention surprisingly ensures a clearlyincreased homogeneity of the insertion of the transition metal in thelithium manganese spinel compared with a doping without the presence ofa boron-oxygen compound, with the result that in particular ahomogeneous uniform insertion of for example aluminium, zinc, etc. inthe 16 d manganese lattice sites takes place. This likewise leads toclearly improved electrochemical properties.

Within the framework of the present invention, an improvement of up toapprox. 20% in the cycle stability was recorded in an electrodecontaining the mixed oxide according to aspects of the inventioncompared with an electrode containing only pure doped or undoped lithiummanganese spinel.

As already briefly stated above, in the lithium manganese spinel of themixed oxide, further manganese lattice sites (16 d position) arepreferably occupied or substituted by at least one metal ion, selectedfrom the group consisting of Al, Mg, Zn, Co, Ni, Cu and Cr or mixturesthereof, with the result that a wide variety of mixed-substitutedhyperstoichiometric lithium manganese spinels can be made available inthe mixed oxide according to aspects of the invention which inparticular can also be optimized with regard to cycle stability. Asalready stated, the presence of the boron-oxygen compound leads to anextremely homogeneous insertion of these metals in the manganese latticesites.

Some of the lithium ions on the 8a tetrahedral sites are furthersubstituted by a metal ion selected from the group consisting of Zn, Mgand Cu, whereby the electrochemical properties of the mixed oxideaccording to aspects of the invention can be adjusted still moreprecisely.

Preferably, the boron-oxygen compound in the mixed oxide according toaspects of the invention is a boron oxide or a boric acid, quiteparticularly preferably B₂O₃ or H₃BO₃. Further representatives of theseclasses of compound that can be used according to aspects of theinvention are (BO)_(x), BO, B₂O, as well as H₂BO₂ and HBO. In likewisepreferred developments of the present invention, borates, i.e. the saltsof orthoboric acid (H₃BO₃) and metaboric acid (H₂BO₂), preferablymonoborates, can also be used. In less preferred embodiments, oligo- andpolyborates can also be used. Alkali metal metaborates M₂BO₂, inparticular Li₂BO₂, are quite particularly preferred. Reference is madein individual cases to Hollemann-Wiberg, Lehrbuch der AnorganischenChemie, de Gruyter, Berlin 102^(nd) edition, 2007, pp. 1104-1110 withregard to the structure and chemistry of the boron-oxygen compounds.

Surprisingly, it was found that the mixed oxide according to aspects ofthe invention has the advantages summarized as follows compared withpure doped or non-doped lithium manganese spinels without the presenceof boron-oxygen compounds:

-   -   The mixed oxide according to aspects of the invention has a        greatly reduced BET surface area of <0.5 m²/g, wherein the BET        surface area correlates with the manganese dissolution in the        electrolyte.    -   The mixed oxide according to aspects of the invention has an        increased tap density (increase from 1.23 g/cm³ to 2.03 g/cm³).    -   The mixed oxide according to aspects of the invention has narrow        reflex widths in the XRD spectrum and large crystallites.    -   The homogeneity of the insertion of the doping metal in the        spinel of the mixed oxide is clearly improved compared with        doped lithium manganese spinel of the state of the art.    -   Electrochemically, an improvement in the cycle stability of the        mixed oxide according to aspects of the invention was recorded        compared with doped lithium manganese spinel of the state of the        art.

The lithium manganese spinel of the mixed oxide contains, as alreadystated above, the lithium on some of the manganese lattice sites(hyperstoichiometry). The doping metal ions are distributed uniformly inthe crystal. This is shown for example by the oxidation state of themanganese and by the X-ray diffraction spectrum (see examples below).

Furthermore, it was also found that as the quantity of boron-oxygencompound in the mixed oxide increases, the quantity of lithium in themixed oxide according to aspects of the invention must also be increasedin order to be able to prepare electrodes with electrochemicallyparticularly advantageous properties. Thus, additional Li must be addedin the molar ratio f=Li/B of 1 up to 4 to 1. Preferably, this ratio is 1up to 3 to 1 and particularly preferably 1.5 up to 3 to 1.

The general formula of the mixed oxide according to aspects of theinvention can be expressed as follows:

[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d) ].b(B_(z)O_(y)H_(u)X_(v))  (I)

-   -   wherein:        -   0≦a<0.1;        -   d<1.2        -   3.5<x<4.5;        -   0.01<c<0.06        -   z=1, 2 or 4        -   y=1, 2, 3 or 7        -   u=0, 1, 2 or 3        -   v=0, 1, 2 or 3        -   0.01<b<0.5    -   M is at least one element from the group of Zn, Mg and Cu;    -   N is at least one element from the group of Al, Mg, Co, Ni, Cu        and Cr;    -   X is at least one element from the group Li, Na, K.

In quite particularly preferred developments, the above-named formulacan be expressed as follows:

[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d))O_(x)].(bB₂O₃ .f*bLi₂O)  (II)

-   -   wherein:        -   0≦a<0.1;        -   d<1.2        -   3.5<x<4.5;        -   0.01<c<0.06        -   0<b<0.05        -   1<f<4;    -   M is at least one element from the group of Zn, Mg and Cu;    -   N is at least one element from the group of Al, Mg, Co, Ni, Cu        and Cr.

The quantity of boron-oxygen compound in the mixed oxide is chosen suchthat the above-defined formula coefficient b is between 0.01 and 0.05,preferably between 0.01 and 0.05, quite particularly preferably between0.0025 and 0.025 and even more preferably between 0.005 and 0.0025. Acoefficient of b=0.001 corresponds to approximately 0.5% by weight ofboron-oxygen compound in the mixed oxide containing lithium manganesespinel.

As already explained above, the lithium manganese spinel of the mixedoxide according to aspects of the invention has a very uniformdistribution of the doping elements N in the crystal lattice.Furthermore, the mixed oxide according to aspects of the invention has avery high phase purity, which is also reflected in the X-ray diffractionspectrum. Preferably, the mixed oxide according to aspects of theinvention has, in an X-ray diffractogram at a diffraction angle of 2θ inthe range from 63.5 to 65°, a signal in which the ratio of the linewidths B₁₀ measured at 10% of the signal height to the line width B₅₀measured at 50% of the signal height is less than 2.0, preferably lessthan 1.8. In particular, the diffraction signal has no shoulder, whichindicates a high phase purity (see also FIGS. 1 and 5).

The hyperstoichiometric lithium is, as already stated, inserted in thelithium manganese spinel according to aspects of the invention onlattice sites of the manganese. This is shown by the degree of oxidationof the manganese. Preferably, the manganese of the spinel in the mixedoxide according to aspects of the invention has a degree of oxidation,defined by cerimetry, in the range from 3.60 to 3.69.

The mixed oxide according to aspects of the invention has comparativelylarge particles, wherein the lithium manganese spinel contained in themixed oxide has few defects. This advantageously results in a small BETsurface area. Preferably, the mixed oxide according to aspects of theinvention has a BET surface area, determined according to DIN 66132, ofless than 4 m²/g.

The mixed oxide according to aspects of the invention has a relativelyhigh particle size. Preferably, the size of the primary crystallites isat least 0.5 μm, preferably <1 μm, most preferably in the range between1 and 3 μm. The size of the primary crystallites can be determined underthe electron microscope. (see also FIG. 4)

A level of doping elements N in which the value for d lies in the rangefrom 0.05 to 0.2 is preferred. A value for d in the range between 0.08and 0.15 is particularly preferred. Preferably, aluminium is used asdoping element N of the mixed oxide according to aspects of theinvention. Alternatively, magnesium is a further preferred dopingelement N, or mixtures of aluminium and magnesium.

So-called high-voltage spinels very strongly doped with Co and Ni forwhich d in the case of Co is preferably 1±0.2 and in the case of nickel0.5±0.1 occupy a special position.

A level of doping elements M which corresponds to a value of a of 0.005to 0.02 is preferred. A particularly preferred doping element M is zinc.

The mixed oxide according to aspects of the invention is very suitableas electrode material for the production of secondary lithium batteries.A subject of the invention is therefore also an electrode, comprising anelectrically conductive support to which the mixed oxide according toaspects of the invention is applied.

A further object of the invention was to provide a process for thepreparation of mixed oxides containing a doped or non-doped lithiummanganese spinel which can be carried out economically and with which amixed oxide containing lithium manganese spinel can be obtained whichcan be used as electrode material in high-performance lithium secondarybatteries.

This object is achieved by a process for the preparation of a mixedoxide containing a mixed-doped hyperstoichiometric lithium manganesespinel, wherein

-   -   at least a lithium component, a manganese component, a        boron-oxygen component, a component containing N, wherein N is        selected from the group of Al, Mg, Co, Ni, Cu and Cr, and/or a        component containing M, wherein M is selected from Zn, Mg and Cu        is provided;    -   a first portion of a solid mixture is prepared by mixing some of        the components in dry, powdery form;    -   a second portion of a liquid mixture is prepared which contains        a portion of the components in dissolved form and/or as a        suspension,    -   wherein the stoichiometric quantities of the components of the        first and second portions are selected such that a mixed oxide        of the following formula results:

[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d))O_(x) ].b(B_(z)O_(y)H_(u)X_(v))

-   -   wherein:        -   0≦a<0.1;        -   d<1.2        -   3.5<x<4.5;        -   0.01<c<0.06        -   z=1, 2 or 4        -   y=1, 2, 3 or 7        -   u=0, 1, 2 or 3        -   v=0, 1, 2 or 3        -   0.01<b<0.5    -   M is at least one element from the group of Zn, Mg and Cu;    -   N is at least one element from the group of Al, Mg, Co, Ni, Cu        and Cr;    -   X is at least one element from the group Li, Na, K;    -   (It is understood that a mixed oxide according to Formula II can        of course also be obtained by a suitable choice of the        quantities of starting materials)        -   the first and second portions of the mixture are mixed;        -   the solvent is drawn off from the mixture obtained,        -   the mixture is calcined at a temperature of more than 300°            C.

With the process according to aspects of the invention for thepreparation of a mixed oxide containing a doped hyperstoichiometriclithium manganese spinel and a boron-oxygen compound, initially at leastone compound each of the elements Li, Mn and boron (here, of course, aboron-oxygen compound) and a compound of a metal N, wherein N isselected from the group of Al, Mg, Co, Ni, Cu and Cr and/or a compoundof a metal M, wherein M is selected from Zn, Mg and Cu, are prepared.The starting components, preferably the lithium or the boron-oxygencomponent, can be prepared in solid form. The boron-oxygen component isone of the boron-oxygen compounds described above.

The solid components are preferably used in pre-ground form. In theexperiments it was shown that the particle morphology of the mixed oxidecan be controlled in a targeted manner through the particle morphologyof a solid and undissolved manganese compound used. In a particularlypreferred embodiment, therefore, a solid and insoluble manganesecompound is used which has previously been brought to the desiredparticle size, which is defined by laser granulometry, of theend-product. The essential aim is to limit the so-called oversizeparticle which causes problems during the processing into thin-filmelectrodes. The D95 value of a solid manganese compound used shouldtherefore lie below 60 μm, preferably below 30 μm. The method ofdetermination will be discussed below.

The compounds are preferably each provided in the form of a metalcompound (thus as a so-called binary compound in which, however, severalanions can also be present, but only one (metal) cation), since thisallows problem-free doping. However, it is also possible to usecompounds which contain several of the metals, thus so-called polynarycompounds with several different (metal) cations.

At least one starting component, namely preferably that of a chemicallyinert doping element such as aluminium, magnesium, zinc, cobalt ornickel, is provided in dissolved form in order to guarantee the finedistribution in the later process steps. The solvent is preferablywater.

The quantity of solvent is chosen such that the components to bedissolved can be completely dissolved. Since the solvent must be removedagain in a later process step, the chosen quantity of solvent is,however, as small as possible. The chosen quantity of solvent should,however, be large enough for it to be able to take up the solidcomponents in the form of a suspension or paste. All of the startingcomponents can also be used in dissolved form in order to create idealconditions for their fine distribution in the later process steps.However, because of the greater solvent requirement and because of thelimitations on the selection of raw materials, this is a less preferredembodiment.

The solid and dissolved components used for the preparation of the mixedoxide containing lithium manganese spinel are preferably provided in theform of their nitrates, acetates, oxides, hydroxides or carbonates.These compounds are, in some cases, very readily soluble in water.

The soluble manganese component is preferably selected from manganesenitrate and manganese acetate.

The solid manganese component is preferably selected from manganesecarbonate, manganese oxide and manganese dioxide.

The boron component is preferably selected from a boric acid, an alkaliborate and a boron oxide. Typical representatives of boric acid areH₃BO₃, H₃BO₂, and of oxides B₂O₃, (BO)_(x), BO₂. Boric acid H₃BO₃ andboron oxide B₂O₃ and also lithium or sodium borates are quiteparticularly preferred.

The named manganese and boron components can be used on their own or ina mixture.

The stoichiometric quantities of the components are selected such thatthey are present in a molar ratio which corresponds to a mixed oxidecomprising a doped lithium-manganese spinel and a boron-oxygen compoundof the following formula:

[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d))O_(x)].b(B_(z)O_(y)H_(u)X_(v))  (I)

-   -   wherein:        -   0≦a<0.1;        -   d<1.2        -   3.5<x<4.5;        -   0.01<c<0.06        -   z=1, 2 or 4        -   y=1, 2, 3 or 7        -   u=0, 1, 2 or 3        -   v=0, 1, 2 or 3        -   0.01<b<0.5    -   M is at least one element from the group of Zn, Mg and Cu;    -   N is at least one element from the group of Al, Mg, Co, Ni, Cu        and Cr;    -   X is at least one element from the group Li, Na, K.

In quite particularly preferred developments, the above-named formulacan be expressed as follows:

[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d))O_(x)].(bB₂O₃ .f*bLi₂O)  (II)

-   -   wherein:        -   0≦a<0.1;        -   d<1.2        -   3.5<x<4.5;        -   0.01<c<0.06        -   0<b<0.05        -   1<f<4;    -   M is at least one element from the group of Zn, Mg and Cu;    -   N is at least one element from the group of Al, Mg, Co, Ni, Cu        and Cr.

The components are then mixed, wherein the mixing can be carried out inany order. In a procedure preferred on process engineering grounds, theliquid partial mixture is introduced first and the solid powdery partialmixture added under stirring, wherein a suspension or paste is obtained.It is possible that a particular element is represented either only inthe solid portion or only in the liquid portion. However, a particularelement can also be distributed over both portions. Thus, it can beadvantageous that the manganese portion is added both as a solidcompound and as a dissolved compound, in order to both control theparticle morphology of the end-product and promote the homogeneousdistribution of the chemically inert doping elements. The mixture can beprepared in a normal mixer.

The mixing conditions are such that essentially no further reduction ofthe insoluble solid components occurs. The components can be mixed attemperatures of 0° to 100° C., but preferably between room temperatureand 50° C. Depending on the mixing intensity of the mixing unit used,the mixing is carried out over a period of a few minutes to severalhours. The mixing process preferably proceeds under chemical conditionsin which compounds of the dissolved chemically inert elements areprecipitated. This can take place for example by altering the pH valuethrough one of the added components or by reacting two dissolvedcomponents or one dissolved and one solid component with each other.However, it is also possible not to precipitate the dissolved chemicallyinert components until the subsequent drying, which is, however, lessadvantageous for their uniform, fine distribution.

When the mixing is concluded, the solvent is drawn off from the mixture.Any process can be used per se. For example, the solvent can be removedby freeze-drying or spray-drying. The solvent can also be removed byheating the mixing container after the mixing and distilling off thesolvent under stirring. The mixture assumes a doughy consistency, withthe result that, as far as possible, measures are taken to prevent themixture sticking to the mixer or on the walls of the mixing container.At the end of the drying process, the mixture can for example also beplaced in a furnace and remaining solvent expelled at increasedtemperature there. The temperatures are chosen such that the evaporationof the solvent is as uniform as possible. Preferably, the drying occursat temperatures of less than 300° C.

When the solvent has been removed, the mixture is calcined in air attemperatures of 300° C. to 800° C. The calcining is preferably carriedout in two stages. The first stage is carried out at a holdingtemperature between 300° C. and 600° C. and for a period of 10 min. to 2hours and serves to expel gaseous decomposition products and form astill little-crystallized and immature spinel compound.

The second stage is carried out as a tempering stage at a holdingtemperature between 600° C. and 800° C., preferably between 740° C. and780° C., and for a period of 1 to 12 hours and serves to mature thespinel compound. The heating rate is as desired, the cooling rate ispreferably between 0.5 K/min. and 5 K/min. The two temperaturetreatments can directly succeed each other or alternatively beinterrupted by an intermediate cooling step and a reduction treatment.

To form a phase-pure and homogeneously doped mixed oxide comprising alithium manganese spinel and a boron-oxygen compound with improvedelectrochemical properties, a fine homogeneous distribution of thechemically inert doping components in the precursor mixture is needed.With the process according to aspects of the invention, this isguaranteed by joint precipitation of these components from the dissolvedstate.

An expensive and energy-consuming high-energy grinding, e.g. in a ballmill, can thereby be dispensed with for example unlike with doped, inparticular boron-doped, aluminium manganese spinels of the state of theart. Moreover, such a process brings with it the risk of contaminationby the rubbings from the grinding tools.

Furthermore, the dissolved and precipitated components are uniformlydistributed in the mixture and are thus also uniformly inserted in thespinel, without excessively high temperatures also having to be usedover an extended period for the thermal treatment of the mixture, as isthe case for example with pure solid-state processes. This increases theprofitability of the process through lower energy costs and a largerthroughput and reduces the losses of volatile lithium during the thermaltreatment. However, a precipitation process followed by temperaturetreatment at a reduced holding temperature and holding time has thedisadvantage of small crystallite sizes and a large specific surfacearea. This increases the attack surface vis-à-vis aggressive electrolyteconstituents.

In order to counter this unfavourable effect, the invention thereforeprovides for an extra addition of a boron-oxygen compound, with theresult that a phase-pure mixed oxide forms. Surprisingly, however, itwas shown that the addition of the boron-oxygen compound in the processaccording to aspects of the invention not only promotes the growth ofthe crystallites but also, as already stated above, the homogeneousdistribution of the remaining doping elements. The distribution,qualifiable by evaluation of the powder X-ray diffraction profiles ofthe end-product in the spinel structure, of the doping elements iscompletely homogeneous in the process according to aspects of theinvention both when using exclusively dissolved starting components andwhen predominantly using solid starting components, as long as aboron-oxygen compound is also added.

Surprisingly, the process according to aspects of the invention leads toa phase-pure mixed oxide in which boron—as already stated above—does notoccupy lattice sites of the manganese (“doping” or “substitution”), butis present as a boron-oxygen compound in the same phase as the dopedlithium manganese spinel. This effect was previously unknown.

Through the process according to aspects of the invention and theformation of mixed oxide, it is possible to prepare X-ray graphicallyphase-pure and homogeneously doped lithium manganese spinels, withoutcomplete dissolution and precipitation of the starting components,without high-energy grinding and without very long temperaturetreatments at high temperature. The process is thereby very economical.In addition, no wastewater and no waste and co-products accumulate.

Moreover, it was surprisingly shown that an increase in boron additionnecessitates a matching increase in the lithium excess(hyperstoichiometry) in order to compensate for negative concomitanteffects.

With the process according to aspects of the invention, the mixed oxidesaccording to aspects of the invention containing doped lithium manganesespinels and a boron-oxygen compound can thus be prepared which have acomparatively large crystallite size and a small BET surface area. Thedoping agents, the metals of the groups formed by N and M, are uniformlydistributed in the spinels on the manganese lattice sites in the spinel.Very uniform crystallites are obtained which have few lattice defects.

A further subject of the invention is a secondary battery, in particulara rechargeable secondary battery, which comprises an electrodecontaining the mixed oxide according to aspects of the invention aselectrode material (“active material”), in particular cathode material.

The invention is described in more detail using examples and referringto the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures show in detail:

FIG. 1: dQ/dE plotted against E for various Co/Al/Li levels of lithiummanganese spinels

FIG. 2: comparison of XRD spectra of mixed oxides according to aspectsof the invention and pure lithium manganese spinels

FIG. 3: the flowchart of the method according to aspects of theinvention

FIG. 4: SEM micrographs of mixed oxides according to aspects of theinvention

FIG. 5: position, reflex width and phase-purity of mixed oxidesaccording to aspects of the invention

FIG. 6: the effect of the addition of borate on the electrochemicalbehaviour

FIG. 7: the manganese dissolution of mixed oxides according to aspectsof the invention compared with pure lithium manganese spinels in mgMn/kg

FIG. 8: the relationship between the BET surface area of mixed oxidecrystals according to aspects of the invention and temperature andlithium hyperstoichiometry

FIG. 9: dQ/dE plotted against E for mixed oxides according to aspects ofthe invention with different lithium hyperstoichiometry

FIG. 10: the discharge capacities of mixed crystals according to aspectsof the invention in the first cycle

FIG. 11: the course of the potential in the first complete cycle ofmixed oxides according to aspects of the invention

FIGS. 12 a-12 b: the course of the potential in the first complete cycleof further mixed oxides according to aspects of the invention

FIG. 13: a scanning electron micrograph of a mixed oxide according toaspects of the invention containing lithium manganese spinel.

GENERAL Analysis Methods:

In the following examples, the following analysis methods were used andimplemented in accordance with the following instructions:

a) Laser granulometry with a Malvern deviceb) BET surface area according to DIN 66132c) Cerimetry (based on U. R. Kunze, Grundlagen der quantitativenAnalyse, page 207, 2^(nd) edition, Thieme Verlag, Stuttgart 1986).Initially, the MnO₂, obtained from a sample of the mixed oxide by e.g.acid decomposition, with a defined excess of Mohr's salt was reacted inacidic solution and the Fe²⁺ that was not consumed was back titratedwith Cer^((IV)) sulphate, whereupon the quantity of MnO₂ and thereforethe analytical content of the sample, consequently the average degree ofoxidation, can be determined via difference calculation.d) XRD measurementApertures (1 mm/1 mm/0.2 mm); radiation: CuK α . . . range: 10-80°;increment: 0.02°; measurement period: 5.5 sec/step

EXPERIMENTAL

The starting products used are commercially available from the followingsuppliers and, unless otherwise indicated, were used as received.

Manganese carbonate (MnCO₃) S grade: Comilog/ErachemLithium hydroxide monohydrate (LiOH*H₂O): Acu PharmaBoric acid (H₂BO₂): Jäkle ChemieAluminium nitrate nonahydrate (Al(NO₂)₂*9H₂O): TropitzschZinc nitrate hexahydrate (Zn(NO₂)₂*6H₂O): PlatoManganese nitrate solution (50% by weight): Coremax/Taiwan

Electrodes were prepared by mixing 70% active material or mixed oxideaccording to aspects of the invention, 20% Super P—Li conductive carbonfrom Timcal and 10% PTFE powder in a mortar until a floc formed. Thefloc was kneaded several times in the mortar before it was rolled out toa thickness of 100 μm in a stainless steel roller press. Round electrodefilms with a diameter of 10 mm were punched out, dried overnight at 200°C. and then arranged in a Swagelok PVDF T-cell for electrochemicalmeasurements. The counter and reference electrodes consisted of alithium metal film and were separated by a glass wool separator filmfrom Whatman. Merck LP30 was used as electrolyte. Thecharge/discharge/test cycles were recorded at a current of C/20 and anominal specific capacity of 120 mA/h in a voltage window between 4.2volts and 2.8 volts.

Comparison Example 1 Preparation of a Lithium Manganese Spinel of theFormula Composition Li[Mn_(1.87)Al_(0.10)Li_(0.03)]₂O₄ with no AddedBoron-Oxygen Compound

The following components were provided as starting materials:

MnCO₃   93% 231.13 g  Al(NO₃)3*9H₂O 98.50% 38.08 g HNO₃   65% 70.77 gLiOH*H₂O 96.30% 44.88 g H₂O   100%   230 g

The manganese carbonate and the aluminium nitrate nonahydrate arestirred into or dissolved in small stages in the previously introducedwater with a laboratory anchor stirrer. The nitric acid is then addedslowly and stirring continued for 10 min. The nitric acid is measuredsuch that approx. ¼ of the manganese carbonate dissolves as manganesenitrate. The laboratory anchor stirrer is set to 40 rpm in order to keepthe suspension floating. The powdery lithium hydroxide monohydrate isthen added and the laboratory anchor stirrer set to 100 rpm for 30 min.in order to compensate for the markedly increasing viscosity. Themedium-brown paste obtained is dried in the laboratory dish at 100° C.for 18 h (amount weighed in 1431 g/amount weighed out 261 g) and thebrown drying product comminuted in the hand mortar. 51.1 g of the dryingproduct was calcined in air in the ceramic crucible at 500° C. for 1 hin the chamber furnace and 32.5 g of a black powder obtained. 9.0 g ofthe calcination product was sintered in air in the ceramic crucible inthe chamber furnace at 750° C. for 12 h, with a heating time of 6 h anda cooling time likewise of 6 h. 8.85 g of sintered product was obtained.

Comparison Example 2 Preparation of an Aluminium- or Cobalt-DopedLithium Manganese Spinel with Lithium Hyperstoichiometry

The samples are prepared by separately introducing manganese nitrate anddopant solutions (aluminium and/or cobalt nitrate) into a receiversolution of LiOH and NH₃. The samples are transferred into a furnacewithout a washing process and predried statically at 160° C. Apre-tempering of the precursor at 500° C. follows. After cooling, theintermediate product is pestled and transformed into the finishedend-product in a second tempering step at different final temperatures(690° C., 730° C. and 770° C.)

Compared with the cobalt-doped product, the crystallite size of thealuminium-doped spinels is clearly smaller.

A more pronounced markedness of the additional stages and lower cyclestability in the examined cycle range were shown in the case of theelectrochemical behaviour of the spinels containing Co/Li. Higher cyclestability was found in the case of the Al/Li-doped sample.

Spinels with different Li/Mn/Co/Al contents were synthesized. Table 1shows the prepared spinels:

TABLE 1 Effect of the dopants (doping elements) Al, Co and Li; thequoted values correspond to the portions of replaced Mn ions in % SampleAl Co Li G1007 0 5 1.5 G1008 0 5 1.5 G1010 5 0 1.5 G1011 5 0 1.5 G1019 05 0 G1020 5 0 0 G1021 0 5 1 G1022 5 0 1 G1028 2.5 2.5 1

The obtained spinels were firstly characterized electrochemically:

Important information about inter alia the cycle stability can be foundin the course of the potential of the charge/discharge curves.Typically, after approximately half charge/discharge, the charge anddischarge curves show a potential step which is attributable tostructural order effects in the spinel lattice. The more pronounced andsharper this jump is, the poorer as a rule the cycle stability of thetested material. This effect already gives an advance indication of thecycle stability of the material.

The same applies for an additional potential step which can occur at 3.1V vs. Li.

The differentials of the potential curves (dQ/dE vs. E) were plotted(FIG. 1) in order to be able to better compare the markedness of thepotential steps. The maxima/minima correspond to the plateaus in thecharge curves. The sharper the peaks here, the more pronounced themarkedness of the steps is.

FIG. 1 shows that the peaks merge more markedly as the aluminium contentin the structure increases, which corresponds to a “blurring” of thepotential curves. This effect is a criterion for the structuralinsertion of the aluminium in the spinel structure. Electrochemically, ahigher cycle stability thus results.

Alongside this, an increase in potential was also established byexchanging cobalt with aluminium.

The consequence of the additional insertion of hyperstoichiometriclithium (in manganese lattice sites) in the aluminium-doped sample is aneven more pronounced overlapping of the peaks. No shift of the peakposition is to be seen.

Example 1 Preparation of a Mixed Oxide According to Aspects of theInvention

The mixed oxide according to aspects of the invention was preparedstarting from a stoichiometric doped lithium manganese spinel (precursorvalue) obtained analogously to comparison example 1. To create thehyperstoichiometry, LiOH and, as boron-oxygen compound, boron oxide inthe form of a mixture of LiOH and B₂O₃ were used. The molar ratio of Lito B was 2 to 1 or f=2.5).

The precursor (5% Al (d=0.1) and 1.5% Li (c=0.03) doping) was mixed withdifferent portions of LiOH and B₂O₃ solutions and reacted according tothe scheme in FIG. 3.

The addition of the boron-oxygen compound, here the borate, affects themorphology, the half-widths of the X-ray reflexes and the BET surfacearea of the Al/Li-doped lithium manganese spinels of the mixed oxideaccording to aspects of the invention.

Even a small addition of borate is already enough to achieve apronounced jump in important properties.

As can be seen in FIG. 4, the crystallite size increases markedly whenborate is added and when the tempering temperature increases. Thelargest crystallites are accordingly achieved at a temperature of 770°C. and when adding 1% by weight (preferably based on the total mixtureof the mixed oxide). However, the BET surface area and, as can be seenin FIG. 5, the half-width of the XRD reflexes clearly decrease.

The examined mixed oxides according to aspects of the invention weresingle-phase (homophase or homeotype) according to the profile of theXRD reflexes, i.e. a complete regular insertion of aluminium into thecrystallite structure had taken place.

Example 2 XRD Spectrum of the Mixed Oxide According to Aspects of theInvention

The XRD spectrum of the mixed oxide according to aspects of theinvention (MO1 1% by weight borate and MO2 0.5% by weight borate),obtained according to Example 1, was compared with the XRD spectra ofAl-doped hyperstoichiometric lithium manganese spinels obtainableaccording to comparison example 1 (M29 and M30) (FIG. 2).

All samples were doped with Li/Al in the ratio 1:5. The aim was to testthe homogeneous insertion of the dopants into the crystal lattice.

In the XRD spectrum, a homogeneously doped single-phase lithiummanganese spinel of the mixed oxide according to aspects of theinvention shows individual “symmetrical” reflexes (MO1 and MO2) (afterKat correction).

The curves of M 29 and M 30 show clearly pronounced shoulders (arrows)(after Kα2 correction). This indicates a multiphase structure or aninhomogeneous distribution of the dopants.

Example 3 Electrochemistry

The presence of a boron-oxygen compound, here the borate, in the mixedoxide according to aspects of the invention obtained according toExample 1 also has a positive electrochemical effect. The cyclestability increases, as can be seen in FIG. 6, as the level of borate inthe mixed oxide increases compared with a pure Al-doped lithiummanganese spinel.

Example 4 Manganese Solution

A reason for the previous lack of cycle stability of pure doped ornon-doped lithium manganese spinels was their decomposition inelectrolytes. In order to examine the relative stability of the mixedoxide according to aspects of the invention compared with adecomposition of the spinel component in an electrolyte, in each case 4g of mixed oxide powder from Example 1 was stored for four weeks at 40°C. with different quantities of borate and an Al-doped lithium manganesespinel from Example 1 in 40 g of LP30 (Merck electrolyte). Theelectrolyte was then analysed for dissolved manganese by means of ICP.The powders tempered at 770° C. were tested.

As can be seen in FIG. 7, the examined samples showed, in the relativecomparison, a decrease in the manganese dissolution as the BET surfacearea increased. It can be seen that a pure lithium manganese spinelwithout the added boron-oxygen compound shows the highest dissolution.The addition of the boron-oxygen compound thus stabilizes the lithiummanganese spinel in the mixed oxide according to aspects of theinvention.

Example 5 Variation of the Lithium Hyperstoichiometry

In further syntheses, the lithium hyperstoichiometry of the spinelcomponent of the mixed oxide was varied progressively with a constant Alcontent.

Chosen dopings were Al 5% Li 1% (d=0.1; c=0.02), Al 5% Li 2% (d=0.1;c=0.04) and Al 5% Li 2.5% (d=0.1; c=0.05). Each of the samples was firstcalcined at 500° C. with and without 1% by weight of an LiOH/B₂O₃mixture (b=0.002; f=2.5) as described and then tempered at 690° C., 730°C. and 770° C.

For the mixed oxides according to aspects of the invention, a clearrelationship between reflex half-widths and lithium hyperstoichiometrywas able to be recorded. As the lithium hyperstoichiometry increased,the reflexes narrowed.

The BET surface area was then correlated with the lithiumhyperstoichiometry:

Due to the influence of the borate, there was a “levelling” of thescatterings of the reflex half-widths depending on the lithiumhyperstoichiometry which occurred in samples from different syntheses(see FIG. 2). The same phenomenon was observed when measuring the BETsurface area of the samples. Here too, a levelling was to be recorded inthe presence of borate (FIG. 8).

The BET values for the samples from the variation of the lithiumhyperstoichiometry are summarized in Table 2.

TABLE 2 BET surface area: Li-hyperstoichiometry variation Li Sample BETm²/g T ° C. (mol % on Mn) G1051B1 0.62 770 1.0 G1051B1 1.07 730 1.0G1051B1 2.5 690 1.0 G1031B2 0.6 770 1.5 G1031B2 2.2 690 1.5 G1052 4.91770 2.0 G1052B1 0.31 770 2.0 G1052B1 0.92 730 2.0 G1052B1 1.66 690 2.0G1053 4.85 770 2.5 G1053B1 0.4 770 2.5 G1053B1 1.04 730 2.5 G1053B1 1.7690 2.5

Overall, as well as the addition of borate and the temperature, thelithium hyperstoichiometry also affects the BET surface area. BET values<1 m²/g were found.

Example 6 Cerimetry

The fact that hyperstoichiometric lithium is inserted into the spinellattice was demonstrated by means of cerimetry using a mixed oxideaccording to aspects of the invention with aluminium-doped manganesespinel. The insertion into the spinel lattice changes the average degreeof oxidation of the manganese ions. Were the lithium present only asfluxing agent (in the form of Li₂O), the average degree of manganese ionoxidation would be +3.56, in the case of a structural insertion, +3.62.

The two structural borderline cases can be represented as follows:

Pure Fluxing Agent

Li₁[Li⁺¹ _(0.029)Al⁺³ _(0.098)Mn^(+x) _(1.874)]O₄ 0.071 Li₂O*wB₂O₃

Mn^(+3.56) Structural Lithium Insertion

Li₁[Li⁺¹ _(0.067)Al⁺³ _(0.094)Mn^(+x) _(1.839)]O₄ w B₂O₃

Mn^(+3.62)

The obtained values which are listed in Table 3 lay in the range from+3.59 to +3.69, which proves the structural insertion.

TABLE 3 Average degree of oxidation, measured by means of cerimetry, ofthe manganese ions in the mixed oxide Li[Li Al Mn]O₄•B₂O₃. Temp.Stoichiometry M^(X+) STDDEV Sample ° C. Li Li Al Mn cerimetric +/− G1031B2 500 1 0.081 0.095 1.824 3.69 0.04 G1031 B2 T1 700 1 0.067 0.094 1.8393.59 0.01 G1031 B2 T2 730 1 0.070 0.095 1.835 3.63 0.01 G1031 B2 T3 6901 0.071 0.094 1.835 3.59 0.01 G1051 B1 T2 730 1 0.076 0.092 1.832 3.600.01 G1053 T1 770 1 0.055 0.094 1.851 3.63 0.01 G1053 B1 T1 770 1 0.1000.093 1.807 3.62 0.02 G1053 T2 730 1 0.053 0.093 1.854 3.59 0.00 G1053B1 T2 730 1 0.097 0.092 1.811 3.60 0.01

The degree of oxidation achieved depends on the synthesis temperature.At higher temperatures, the degree of oxidation was somewhat lower. Thisfinding correlates well with the above-described structuraltemperature-related shift of the reflex layers in the XRD spectrum.

In FIG. 9, dQ/dE vs. E of materials of different mixed oxides accordingto aspects of the invention with different lithium hyperstoichiometriesare plotted.

Example 7 Electrochemical Examination

FIG. 10 compares the discharge capacities of samples of mixed oxidesaccording to aspects of the invention in the first cycle. Thetheoretical value was calculated from the analytically determined spinelstoichiometry. The measured values lie between 117 mAh/g for a purelylithium-doped (hyperstoichiometric) sample (EXM 1666) and 96 mAh/g forone doped with Li/Al/Zn.

The theoretical maximum capacity decreases from 129 mAh/g for EXM 1663to 109 mAh/g for EXM1666. The measured values show the same tendency ata lower level.

The reference samples showed comparatively small capacities, which canbe attributed to the increased lithium insertion.

FIG. 11 shows in the overview the courses of the potential of thereference samples in the first complete cycle. In FIG. 12, the emphasisis on potential ranges which gave initial indications of the cyclebehaviour of the samples. Both the occurrence of an additional potentialstep at 3.2 V and a well-pronounced step at half charge/discharge areconsidered a feature of poor cycle stability:

-   -   The potential plateau at 3.2 V (FIG. 12 a, left-hand side) is        clearly pronounced for the Al-free sample, still difficult to        spot for the sample doped with 3% Al and no longer present in        the models doped with 5% Al.    -   The markedness of the stage of potential at half        charge/discharge decreases in the order EXM1663 after EXM1666.

Example 8 Industrial-Scale Preparation of a Mixed Oxide ContainingLithium Manganese Spinel According to Aspects of the Invention of theComposition Formula(Li_(0.99)Zn_(0.01))[Mn_(1.87)Al_(0.1)Li_(0.03)]O₄.(0.9B₂O₃, 2*0.9Li₂O)

In each of several stirring mixtures, the following quantities of rawmaterials were used:

MnCO₃ 4.360 kg 93.50% Mn(NO₃)₂ sol. 2.843 kg   50% Al(NO₃)₃ * 9H₂O 0.867kg   101% Zn(NO₃)₂ * 6H₂O  70.2 g   99% H₂O (dist.) 3.530 kg LiOH * H₂O1.063 kg   55% Boric acid 12.80 g 100.3%

The water, the manganese nitrate solution, the aluminium nitratenonahydrate and the zinc nitrate hexahydrate were each placed in a 121plastic bucket and mixed with a Pendraulik stirrer with stirring diskand dissolved completely. The finely-powdered boric acid and themanganese carbonate pre-ground to a D95 value of 27 μm were then added.The Pendraulik stirrer was first set to level 3 in order to keep thestill highly liquid suspension homogeneous. The powdery lithiumhydroxide was then added within 1 min. and stirred at level 4 for afurther 15-20 min. The viscosity of the suspension increased rapidly.

The stirring mixtures were combined in a stirred receiver container andthe still pumpable suspension was dried in a Storck Bowen spray drierwith two-fluid nozzle and spraying from above in the cocurrent processat an air-entry temperature of 550° C. and an air-exit temperaturebetween 140° C. and 145° C. A brown powder with a bulk density of 1011g/l was obtained.

The drying product obtained in this way still contains nitrates and wascalcined on stainless steel tray sheets in a continuous belt furnacewith an attached exhaust-gas cleaning unit for nitrous vitriol gases at450° C. and an average residence time of 1 h in the heating zone.Surprisingly, small quantities of nitrous vitriol gases formed and afine black powder with a bulk density of 823 g/l was obtained. Thecalcination product was then tempered in air in an Alsint-ceramiccrucible in the chamber furnace at 770° C. for 12 h with a heating timeof 6 h and a cooling time of 12 h.

The bluish black tempering product was ground on an Alpine AFG100 airseparator mill with ceramic separator wheel and ceramic milling airnozzles 3.0 mm in diameter at a separator speed of 5500 rpm.

The milled product collected in the centrifugal cyclone separator had abulk density of 1045 g/l and an apparent density of 1713 g/l. FIG. 13shows a scanning electron micrograph of the product. The chemicalcomposition determined by means of ICP was:

Li  3.9% Mn 59.7% Al  1.6% Zn 3800 mg/kg  B 600 mg/kg S 0.23% Na 920mg/kg

The particle-size distribution was monomodal and characterized by thefollowing parameters:

D₁₀=2.5 μm D₅₀=10.8 μm D₉₀=20.7 μm D₁₀₀=35.6 μm

Further product properties were:

pH value: 9.5residual moisture content: 0.26% by weight (Karl-Fischer method)BET surface area: <1 m²/gmicropore volume: <0.001 cm³/g (ASAP 2010, Micromeritics)cubic lattice constant a=8.210 Å (according to X-ray diffraction)

In the electrochemical cycle test, the mixed oxide containing lithiummanganese spinel according to aspects of the invention shows a dischargecapacity of 105 mAh/g and a cycle loss of less than 0.1% per cyclemeasured in a half-cell of the LMS//LiPF6-EC-DMC//Li type.

1. Mixed oxide containing a) a mixed-substituted lithium manganesespinel in which some of the manganese lattice sites are occupied bylithium ions and b) a boron-oxygen compound.
 2. Mixed oxide according toclaim 1, wherein in the doped lithium manganese spinel further manganeselattice sites are occupied by a metal ion N, selected from the groupconsisting of Al, Mg, Zn, Co, Ni, Cu and Cr.
 3. Mixed oxide according toclaim 2, wherein some of the lithium ions on the 8a tetrahedral sitesare substituted by a metal ion from the group consisting of Zn, Mg andCu.
 4. Mixed oxide according to claim 2, wherein the boron-oxygencompound is a boron oxide, a boric acid or salts thereof.
 5. Mixed oxideaccording to claim 4, wherein the following formula results for thecomposition:[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d) ].b(B_(z)O_(y)H_(u)X_(v))wherein: 0≦a<0.1; d<1.2 3.5<x<4.5; 0.01<c<0.06 z=1, 2 or 4 y=1, 2, 3 or7 u=0, 1, 2 or 3 v=0, 1, 2 or 3 0.01<b<0.5 M is at least one elementfrom the group of Zn, Mg and Cu; N is at least one element from thegroup of Al, Mg, Co, Ni, Cu and Cr; X is at least one element from thegroup Li, Na, K.
 6. Mixed oxide according to claim 5, wherein the X-raydiffractogram of the mixed oxide has a signal at a diffraction angle 2θin the range from 63.5 to 65°, in which the ratio of the line widths B₁₀measured at 10% of the signal height to the line width B₅₀ measured at50% of the signal height is less than 2.0, preferably less than 1.8. 7.Mixed oxide according to claim 6 with a BET surface area of less than 4m²/g.
 8. Mixed oxide according to claim 7, wherein the size of theprimary crystallites of the mixed oxide, measured as D₅₀, is at least0.5 μm, preferably greater than 1 μm.
 9. Electrode, comprising anelectrically conductive support to which a mixed oxide according toclaim 1 is applied.
 10. Secondary battery with an electrode according toclaim
 9. 11. Process for the preparation of a mixed oxide containing amixed-doped hyperstoichiometric lithium manganese spinel, wherein atleast a lithium component, a manganese component, a boron component, acomponent containing N, wherein N is selected from the group of Al, Mg,Co, Ni, Cu and Cr, and/or a component containing M, wherein M isselected from Zn, Mg and Cu is provided; a first portion of a solidmixture is prepared by mixing some of the components in dry, powderyform; a second portion of a liquid mixture is prepared which contains aportion of the components in dissolved form and/or as a suspension,wherein the stoichiometric quantities of the components of the first andsecond portions are selected in such a way that a mixed oxide of thefollowing formula results:[(Li_(1−a)M_(a))(Mn_(2−c−d)Li_(c)N_(d) ].b(B_(z)O_(y)H_(u)X_(v))wherein: 0≦a<0.1; d<1.2 3.5<x<4.5; 0.01<c<0.06 z=1, 2 or 4 y=1, 2, 3 or7 u=0, 1, 2 or 3 v=0, 1, 2 or 3 0.01<b<0.5 1<f<4; M is at least oneelement from the group of Zn, Mg and Cu; N is at least one element fromthe group of Al, Mg, Co, Ni, Cu and Cr; X is at least one element fromthe group Li, Na, K; the first and second portions of the mixture aremixed; the solvent is drawn off from the mixture obtained, the mixtureis calcined at a temperature of more than 300° C.
 12. Process accordingto claim 11, wherein the calcining is carried out in at least two steps,wherein calcination is at temperatures of 300 to 600° C. in a first stepand at temperatures of 600 to 900° C. in a second step.
 13. Processaccording to claim 11, wherein the chosen D₅₀ value of the firstcomponent is smaller than 30 μm.
 14. Process according to claim 13,wherein before the mixing the manganese component is ground so finelythat a D₉₅ value measured by laser granulometry of less than 30 μm isset.
 15. Process according to claim 11, wherein water is used as solventfor the preparation of the second portion of the mixture.
 16. Processaccording to claim 15, wherein the second portion of the mixturecontains a portion of the manganese component and/or of component N. 17.Process according to claim 16, wherein the first part of the mixture isstirred into the second part of the mixture.
 18. Process according toclaim 11, wherein the components are provided in the form of theirnitrates, acetates, oxides, hydroxides and/or carbonates.
 19. Processaccording to claim 14, wherein the manganese component is selected frommanganese carbonate, manganese oxide and manganese dioxide.
 20. Processaccording to claim 11, wherein the values chosen for f, b and d are1.5<f<3, 0.0025<b<0.025, 0.05<d<0.2.
 21. Process according to claim 20,wherein the values chosen for f, b, d and a are 1.5<f<3, 0.0025<b<0.025,and 0.08<d<0.15, and 0.005<a<0.02.
 22. Process according to claim 11, inwhich N is selected from Al and Mg, and M is selected from Zn.